1. Field of the Invention
The invention relates to a powder based on polymer-coated fillers which has advantages in terms of the stability of the production process, and density, to the use of the said powder in shaping processes, and also to mouldings produced by a layer-by-layer process by which regions of a powder layer are selectively melted, with use of the said powder. After cooling and solidification of the regions previously melted layer-by-layer, the moulding can be removed from the powder bed. The mouldings according to the invention moreover exhibit less susceptibility to warpage than conventional mouldings.
2. Description of the Related Art
A task frequently encountered in very recent times is the rapid provision of prototypes. Particularly suitable processes are those which are based on pulverulent materials and in which the desired structures are produced layer-by-layer through selective melting and solidification. Supportive structures for overhangs and undercuts can be omitted here, because the powder bed surrounding the molten regions provides sufficient support. Nor is there any need for the subsequent operation of removing supports. The processes are also suitable for producing short runs.
The selectivity of the layer-by-layer process here can be provided by way of example by applying susceptors, absorbers, or inhibitors, or by masks, or by way of focussed introduction of energy, for example through a laser beam, or by way of glass fibres. The energy is introduced by way of electromagnetic radiation.
A process which has particularly good suitability for the purpose of rapid prototyping is selective laser sintering. In this process, plastics powders are briefly irradiated selectively in a chamber by a laser beam, and the powder particles which encounter the laser beam therefore melt. The molten particles coalesce and rapidly resolidify to give a solid mass. This process can provide simple and rapid production of three-dimensional products by repeated irradiation of a succession of freshly applied layers.
The laser sintering (rapid prototyping) process for producing mouldings from pulverulent polymers is described in detail in the U.S. Pat. No. 6,136,948 and WO 96/06881. A wide variety of polymers and copolymers is claimed for the said application, examples being polyacetate, polypropylene, polyethylene, ionomers and polyamide.
Other processes with good suitability are the selective inhibition bonding (SIB) processes described in WO 01/38061, and a process described in EP 1 015 214. Both processes operate with large-surface-area infrared heating for melting of the powder. The selectivity of the melting process is achieved in the first case by applying an inhibitor, and in the second process it is achieved by a mask. DE 103 11 438 describes another process. In this, the energy required for the fusion process is introduced through a microwave generator, and the selectivity is achieved by applying a susceptor.
Other suitable processes are those operating with an absorber which is either present in the powder or is applied by ink-jet processes, as described in DE 10 2004 012 682.8, DE 10 2004 012 683.6 and DE 10 2004 020 452.7.
The rapid prototyping or rapid manufacturing processes mentioned (RP or RM processes) can use pulverulent substrates, in particular polymers, preferably selected from polyesters, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, or a mixture thereof.
WO 95/11006 describes a polymer powder which is suitable for the laser sintering process and which, when melting behaviour is determined by differential scanning calorimetry with a scanning rate of from 10 to 20° C./min, exhibits no overlap of the melting and recrystallization peak, has a degree of crystallinity of from 10 to 90%, likewise determined by DSC, has a number-average molecular weight Mn of from 30 000 to 500 000, and has a Mw/Mn quotient in the range from 1 to 5.
DE 197 47 309 describes the use of a nylon-12 powder which has increased melting point and increased enthalpy of fusion and which is obtained by reprecipitation of a polyamide previously produced through ring-opening and subsequent polycondensation of laurolactam. This is a nylon-12.
DE 10 2004 003 485 describes the use of particles with at least one cavity for use in processes that build layers. All of the particles comprise at least one cavity, and the particles comprising the cavity are melted by introduction of electromagnetic energy. The powder particles described have a thin surface layer.
DE 102 27 224 describes a granulated material which is intended for 3D binder printing and which is composed of particles provided with a surface layer comprising a non-polar external area. The surface layer of the powder particles described is, however, thin.
In conventionally known methods, powders such as described above are sometimes mixed with other particles for reinforcement, e.g. metal particles, glass particles or TiO2 particles. However, a disadvantage encountered in these processes is that the handling of powder mixtures of this type often leads to demixing phenomena, and the mechanical properties that the reinforcing material is intended to achieve therefore sometimes vary. The regions where the proportion of filler is too high become very brittle and therefore unusable, and the regions comprising too little filler are softer than intended. The demixing derives from the different density of the polymer particles and of the filler, and tends to be apparent to some extent during any transport of the powder mixture and during its handling. In particular if the handling of the powder is automated in the rapid manufacturing process, it is difficult to control deviations in the properties of the components produced.
WO 2007/051691 describes processes for producing ultra-fine powders based on polyamides, by precipitating polyamides in the presence of inorganic particles, where a suspension is used with inorganic particles suspended in the alcoholic medium, where the d50 median size of the inorganic particles is in the range from 0.001 to 0.8 μm. Fine polyamide powders were obtained here, and, because of their small size, the inorganic particles have uniform distribution in the composite particles here. The process was aimed at achieving colouring of the powder and of the moulding formed therefrom. The measure does not alter the mechanical properties of the moulding.
It was an object of the present invention to eliminate the problem of the demixing phenomenon and to achieve an improvement in the consistency of mechanical properties in the moulding, preferably strengthening of the moulding, and providing flame retardancy and/or an improvement in thermal conductivity in the moulding, where these improvements are intended to be achieved with the reinforcing material.